1. Field of Invention
The present invention relates to voltage-controlled oscillators which are used in, for example, wireless communication apparatuses, video apparatuses, and measuring instruments, and more particularly relates to a voltage-controlled oscillator, an IC chip for a voltage-controlled oscillator, a resistance regulating apparatus, and a resistance regulating method, which require precise frequency variable characteristics.
2. Description of Related Art
An example of a conventional general voltage-controlled oscillator is shown in FIG. 19. As shown, a voltage-controlled piezoelectric oscillator 200 includes an input resistor Ri connected to a frequency control terminal VC, a piezoelectric resonator X, a variable capacitance diode (varactor) Cv, an oscillation-amplitude adjusting resistor Rs, and an oscillator circuit 100.
In the voltage-controlled piezoelectric oscillator 200 of FIG. 19, application of a control voltage Vc to the frequency control terminal VC changes the capacitance of the varactor Cv, thus adjusting an oscillation frequency f generated by resonance with the piezoelectric resonator X.
In the voltage-controlled piezoelectric oscillator 200, as shown by frequency variable characteristics indicated by dotted lines in FIG. 20(a), the voltage value of the control voltage Vc is changed within a predetermined range (range from Vcxe2x88x92 to Vc+ with respect to a reference control voltage Vc0), and hence the oscillation frequency f of an oscillation signal S outputted from an output terminal OUT is changed linearly within a predetermined range (range from xe2x88x92100 to +100[ppm] with respect to a reference frequency f0, which corresponds to the reference control voltage Vc0). In FIG. 20(a), the control voltage Vc is plotted along the x-axis, and frequency deviation xcex94f of the oscillation frequency f (frequency deviation xcex94f=(fxe2x88x92f0)/f0) is plotted along the y-axis, where f0 is the reference oscillation frequency.
The piezoelectric resonator X can be replaced by the equivalent circuit shown in FIG. 21. Even when at the same frequency, when the piezoelectric resonator X is formed of, for example, a crystal strip, the equivalent circuit constants may vary according to a difference in shape, such as a rectangular shape or a disk shape, a difference in size, or the like.
For example, when the resistance of resistor R1 in the equivalent circuit is smaller than the resistance of the normal piezoelectric resonator X, the impedance of an oscillator loop consisting of the varactor Cv, the piezoelectric resonator X, the oscillation-amplitude adjusting resistor Rs, and oscillation capacitors C1 and C2 is reduced, whereas the oscillation amplitude of the oscillation signal is increased. In such a case, when the control voltage Vc is set to a low potential, a desired frequency may not be achieved since frequency variable characteristics are not linear.
Specifically, as indicated by solid lines in FIG. 20(a), when the control voltage Vc applied to the frequency control terminal VC is changed from a high potential (Vc+) to a low potential (Vcxe2x88x92), the linearity of the frequency variable characteristics is distorted in the vicinity of the low potential.
As shown in FIG. 20(b), the cause of this is that when the control voltage Vc approaches a low potential, the capacitance of the varactor Cv is increased (the impedance is reduced), and the negative side of the oscillation amplitude is clipped by a forward voltage (approximately xe2x88x920.7 V) of the varactor Cv. FIG. 20(b) shows time along the x-axis and a potential of the oscillation amplitude generated in the oscillator loop along the y-axis.
When the negative side of the oscillation amplitude is clipped by the forward voltage, the control voltage Vc applied to the varactor Cv is greater than the control voltage Vc applied to the frequency control terminal VC by an average potential (see arrow b in FIG. 20(b)). Hence, there is a difference between the control voltage Vc applied to the varactor Cv and the actual potential of the varactor Cv. This results in distortion of the linearity of the frequency variable characteristics as the control voltage Vc approaches Vc-(GND).
In the voltage-controlled piezoelectric oscillator 200, when the components other than the piezoelectric resonator X and the varactor Cv are formed as an IC chip, the oscillation-amplitude adjusting resistor Rs has a fixed value. It is therefore necessary to provide a plurality of IC chips in order that each IC chip has the oscillation-amplitude adjusting resistor Rs with a different resistance for each piezoelectric resonator X with a different oscillation frequency f. As a result, the cost of producing is increased.
In view of the above circumstances, it is an object of the present invention to provide a voltage-controlled oscillator, an IC chip for a voltage-controlled oscillator, a resistance regulating apparatus, and a resistance regulating method, which are capable of improving the linearity of frequency variable characteristics and increasing the range of frequency that can be varied by a control voltage.
The present invention relates to a voltage-controlled oscillator including a piezoelectric resonator having a variable reactance element connected in series to an input side of the piezoelectric resonator, an oscillator circuit connected to a variable reactance element, and a resistor circuit provided in an oscillator loop including the piezoelectric resonator, the variable reactance element, and the oscillator circuit, the resistor circuit including a plurality of oscillation-amplitude adjusting resistors which are selectively inserted.
The present invention further provides that the resistor circuit includes a switch for connecting or disconnecting the oscillation-amplitude adjusting resistors based on a resistor connection control signal. Further, the switch can include a semiconductor switch.
Additionally, the present invention can provide a resistor information memory for storing resistor connection control data, and a control circuit for storing the resistor connection control data beforehand in the resistor information memory, based on an adjusting resistor control signal which is supplied from the outside, and for outputting a resistor connection control signal for controlling the switch based on the adjusting resistor control signal or the resistor connection control data.
The above-described resistor circuit can be such that one end of each of the oscillation-amplitude adjusting resistors is connected to the oscillator loop and the other end includes a bonding pad.
The present invention may further provide a capacitor circuit connected to the variable reactance element, the capacitor circuit including a plurality of selectable-capacitance elements, and a capacitance connecting circuit for connecting to the variable reactance element a desired capacitive element from among the selectable-capacitance elements.
The piezoelectric resonator can be a crystal resonator. Further, components other than the piezoelectric resonator and the variable reactance element can be formed as an IC chip, wherein in the IC chip, the piezoelectric resonator, and the variable reactance element can be integrally mold-sealed. Additionally, the IC chip, the piezoelectric resonator, and the variable reactance element can be contained in a single package.
The present invention further provides an IC chip for a voltage-controlled oscillator, the voltage-controlled oscillator including a piezoelectric resonator, a variable reactance element connected in series to the input side of the piezoelectric resonator, and an oscillator circuit connected to the variable reactance circuit. The IC chip is characterized by including a resistor circuit provided in an oscillator loop including the piezoelectric resonator, the variable reactance element, and the oscillator circuit, the resistor circuit including a plurality of oscillation-amplitude adjusting resistors which are selectively inserted.
The present invention further provides that the resistance regulating apparatus can include a frequency detecting device for sequentially selecting one from among oscillation-amplitude adjusting resistors and detecting the frequency of an oscillation signal in a state in which a predetermined reference control voltage is gradually applied to a control voltage terminal, and an oscillation-amplitude adjusting resistor confirming device for confirming the desired oscillation-amplitude adjusting resistor which corresponds to a predetermined oscillation frequency, based on the detected frequency of each oscillation-amplitude adjusting resistor.
The present invention further provides that the resistance regulating apparatus can include a frequency detecting device for sequentially selecting one from among oscillation-amplitude adjusting resistors and detecting the frequency of an oscillation signal in a state in which a predetermined reference control voltage is gradually applied to a control voltage terminal, a frequency variable characteristic generating device for generating a frequency variable characteristic of each oscillation-amplitude adjusting resistor, based on the detected frequency of each oscillation-amplitude adjusting resistor, and an oscillation-amplitude adjusting resistor confirming device for confirming the desired oscillation-amplitude adjusting resistor which corresponds to a predetermined oscillation frequency, based on the generated frequency variable characteristic of each oscillation-amplitude adjusting resistor.
The present invention provides that the resistance regulating method can include a frequency detecting step of sequentially selecting one from among oscillation-amplitude frequency adjusting resistors and detecting the frequency of an oscillation signal in a state in which a predetermined reference control voltage is gradually applied to a control voltage terminal, and an oscillation-amplitude adjusting resistor confirming step of confirming the desired oscillation-amplitude adjusting resistor which corresponds to a predetermined oscillation frequency, based on the detected frequency of each oscillation-amplitude adjusting resistor.
The present invention further provides that the resistance regulating method can further include a frequency detecting step of sequentially selecting one from among oscillation-amplitude adjusting resistors and detecting the frequency of an oscillation signal in a state in which a predetermined reference control voltage is gradually applied to a control voltage terminal, a frequency variable characteristic generating step of generating a frequency variable characteristic of each oscillation-amplitude adjusting resistor, based on the detected frequency of each oscillation-amplitude adjusting resistor, and an oscillation-amplitude adjusting resistor confirming step of confirming the desired oscillation-amplitude adjusting resistor, based on the generated frequency variable characteristic of each oscillation-amplitude adjusting resistor.